The Arsenal manager expressed his surprise that Chelsea should do business with a Premier League rival, albeit one that is flagging this season, and he warned his Stamford Bridge counterpart, José Mourinho, that he could live to regret the decision in the longer term.

Wenger suggested that Chelsea considered the £40m sale, which is close to being finalised, as beneficial to their title challenge, in that Mata stands to be able to help United in their upcoming fixtures against Arsenal and Manchester City, who sit first and second in the table, ahead of Chelsea in third.

Chelsea played their second league match of the season against United at Stamford Bridge last Sunday, which they won 3-1 – Mata was an unused substitute – and, afterwards, Mourinho said with a smile that he now hoped United "beat all of them [the top clubs] to finish top four".

"He [Mourinho] sums up what Chelsea's thinking is," Wenger said, with regard to Mata. Wenger stressed that Chelsea had done nothing wrong but it was clear that he felt they had sought to manipulate the market.

"I am surprised because Juan Mata is a great player and they sell a great player to a direct opponent," Wenger said. "It opens [up] again a little bit the opportunity of this transfer market because Chelsea had already played twice against Man United so they don't play again. They could have sold him last week [before the United game].

"It opens, at least, the opportunity of the dates of this transfer window. Some teams have already played twice against one opponent and some others, not. If you want to respect the fairness for everybody exactly in the game, that should not happen. I just think you would want everybody on the same level. I can understand completely what Chelsea is doing. They do not do the rules but, maybe, the rules should be a bit more adapted to more fairness."

Wenger came in for criticism for selling Robin van Persie in the summer of 2012 to United. "Yes," he said. "That's why I was surprised [about the Mata deal]. In our defence, we were in a more explicable situation because Van Persie had only one year to go. He was 29 and had played here for a long time. It was not a club that I wanted to sell Van Persie to. I wanted to sell him to Juventus. But he had decided to go to Man United and, at the end of the day, you need the agreement of the three parties."

Wenger said that Mata was a player he "loved" – he tried to sign him from Valencia before Chelsea got him in 2011. The Arsenal manager does not see United as being in decline and he believes that Chelsea might come to regret moving him to Old Trafford.

"They could," he said. "I don't think about Man United like you think about Man United. It's a short moment in their history and I don't see Man United as a club in decline. This can happen to any big club but they will come up again, don't worry."

Wenger did take into account the size of the fee that United will pay for Mata – a club record £40m – and he veered from his slightly conspiratorial theme when he said that the money was the most persuasive reason for Chelsea's sale.

"That is the first time I come to that conclusion with Chelsea," Wenger said. "It means somewhere that Chelsea changes philosophy and they want to get on line with financial fair play. That is a good thing. They sold [Kevin] De Bruyne [to Wolfsburg for €20m]. And Mata. They lost £50m last year, disguised. Maybe it's more. If they get £37m now for Mata … "

Arsenal's manager Arsène Wenger does not agree with the January transfer window. Photograph: Stephen Pond/PA

United's need for reinforcements in midfield became even more apparent with the news that Michael Carrick is expected to be ruled out for up to four weeks with an ankle injury. The midfielder sustained possible ligament damage in Wednesday night's Carling Cup game against Sunderland but the joint was too swollen to be scanned on Thursday. Carrick has recently returned from six weeks out following surgery on his achilles.

Arsenal take on Coventry City in the FA Cup at home on Friday night and Wenger said that he would name a strong team in order to maintain the club's momentum. But he feels that something has to change on football's financial side."If you imagine 20 clubs in the Premier League with 20 multi-billionaires," Wenger said. "At some stage, they have to sit down and say, 'Look, we are killing each other just for the sake to pay higher wages, or do we make a rule that makes sense?'

"It will happen in the longer term, there's no other way. We had only one or two billionaires at the start but once we have 10, they have to respect some rules or they kill each other and it's endless."

About this article

Arsène Wenger questions motives of Chelsea decision to sell Juan Mata

This article was published on
the Guardian website
at 18.38 EST on Thursday 23 January 2014.
A version appeared on p1 of the Sport section of the Guardian
on Thursday 23 January 2014.
It was last modified at 00.30 EDT on Tuesday 3 June 2014.
It was first published at 18.00 EST on Thursday 23 January 2014.